1. Field of the Invention
This invention relates to a surface-treated aluminum or aluminum alloy material used in construction materials, heat-exchanger fin materials, printed circuit boards, can tub materials and other materials. It also relates to a method for its surface treatment.
2. Description of Related Art
A number of techniques have been proposed as methods for the surface treatment of aluminum or aluminum alloy materials for the purposes of anticorrosion, prevention of color changes, improvement in adhesion of coatings, appearance beautification and so forth.
Among them, relatively effective techniques can be chiefly exemplified by the following.
(1) Japanese Patent Laid-open No. 50-86540: An aluminum substrate is subbing-treated with an aqueous alkali silicate solution, and thereafter treated with a vinylidene chloride resin to form an anticorrosive coating.
(2) Japanese Patent Laid-open No. 64-34731: An aluminum sheet is pretreated with a weak alkali to degrease its surface, and thereafter treated in an aqueous solution containing an oxidizing agent to form a boehmite coating.
(3) Japanese Patent Laid-open No. 59-133382: An aqueous alkaline solution comprising about 3 to 6 g/lit. of a chelating agent is sprayed to carry out cleaning.
(4) Japanese Patent Laid-open No. 1-212775: Subbing treatment is applied by etching in an aqueous acidic solution of pH 2 or less containing chloride ions, followed by etching in an aqueous alkaline solution of pH 13 or more.
(5) Japanese Patent Laid-open No. 2-97700: Pretreatment for degreasing is applied in an aqueous solution to which a surface active agent has been added, and thereafter anodic polarization electrolysis and cathodic polarization electrolysis are alternately carried out. Every method, however, involves some problems.
The method (1) has the problem that the treatment tends to become uneven when the treatment with an alkali silicate is applied, making it difficult to obtain homogeneous coatings. In the method (2), probably because of the treatment for degreasing with a weak alkali, defective coatings and blackening may occur during the subsequent boehmite treatment and coating treatment, and such problems remain unsettled.
In the method (3), because of an unsatisfactory power for removing oxide layers by the aid of only a chelating agent, the blackening of material surfaces can not be completely avoided. The method (4) is a method capable of completely removing oxide layers, but requires two-stage treatment, which makes productivity poor to cause a cost increase. Moreover, because of smuts that may occur during the process, the problems of defective coatings and so forth may be caused in the subsequent coating and chemical conversion. The method (5) tends to give an unsatisfactory result on the degreasing pretreatment using the surface active agent, which causes uneveness in electrolysis. In this case, in order to obtain uniform treated surfaces, electric power is required in a large quantity, leading to a cost increase. This method has also the problem that it can not be applied to articles having complicated shapes, showing a difficulty in carrying out the surface treatment.
Difficulties such as poor anticorrosion, color changes, poor adhesion of coatings and so forth occurring in surface-treated aluminum materials are ascribable to a surface oxide layer. This surface oxide layer is thin, but is a porous layer containing a rolling oil or the like and has a large number of cracks, so that it has a poor wettability to coating materials to cause a poor adhesion of coatings and poor anticorrosion.
Any surface treatment carried out without complete removal of the surface oxide layer results in incorporation of components of this surface oxide layer into a hydrated oxide (boehmite) layer or an anodic oxide layer, so that a black or brown film is formed to cause damage of surface appearance. In the treatment by anodic oxidation or chemical conversion (chromating or the like), the dissolution of aluminum surface has influence on the initiation of film forming reaction, and hence any unhomogeneous oxide layer adhering to the surface causes non-uniform reaction, resulting in occurrence of defective coatings.
Thus, in order to obtain highly anticorrosive surface-treated aluminum materials having a beautiful surface appearance, any oxide layer produced during forging, rolling and forming must have been completely removed.
The extent to which this oxide layer has been removed can be measured by spectroscopic means such as EPMA (electron probe microanalysis) and ESCA (electron spectroscopy for chemical analysis) or measuring means such as SEM (scanning electron microscopy) and TEM (transmission electron microscopy). These, however, take time and labor, and moreover are not feasible for non-destructive measurement of the quantity of any remaining oxide layer except for small utensils. Accordingly, the setting of conditions for pretreatment has been hitherto often relied on experience and intuition, so that the treatment for the removal of oxide layers may be excessively carried out or, in reverse, the chemical conversion may be carried out without sufficient treatment for the removal of oxide layers, to often cause difficulties.
Thus, it is necessary to provide an index or guideline that enables simple measurement and also is precise. It, however, is not necessarily important to measure the quantity itself of a remaining oxide layer. After all, there may be at least a simple physical quantity that reflects the quantity of a remaining oxide layer.